Both local and remote alarms responding to fluctuations in water pressure are presently incorporated in sprinkler systems to indicate the activation of any part of the system and thereby provide a warning of a fire. It is common practice to provide in a readily accessible position a master stop valve to shut off the water supply to the sprinklers when the fire is extinguished. However, due to their ready accessibility unauthorised tampering with these valves does occur and the danger exists that the stop valves may be in a closed condition at a time when a fire occurs, and not only is the sprinkler system inoperative but also the alarm.